There has been proposed an amplification type solid-state imaging device configured to provide an amplification function to each of a plurality of pixels and perform a read operation using a scanning circuit, i.e., a CMOS image sensor. In the CMOS image sensor, a photoelectric conversion section, an amplification section, a pixel selection section and a reset section are formed in one pixel, wherein three MOS transistors are used for the sections other than the photoelectric conversion section composed of a photodiode (see, for example, the Patent Publication 1). In other words, the conventional CMOS image sensor consists of four elements. The CMOS image sensor is operable to store electric charges generated by the photoelectric conversion section composed of a photodiode, and, after amplifying the stored electric charges through the amplification section, read the amplified electric charges using the pixel selection section.
FIG. 1 shows a unit pixel of the conventional CMOS image sensor. In FIG. 1, the reference numeral 001 indicates a photoelectric-conversion photodiode. The reference numerals 006, 007 and 008 indicate an amplification transistor, a reset transistor and a selection transistor, respectively. The reference numerals 004, 002, 003, 005 and 009 indicate a signal line, a pixel-selection clock line, a reset clock line, a power supply line, and a reset power supply line, respectively. The unit pixel of the conventional CMOS image sensor has the photodiode and the three MOS transistors in a plane. Thus, it is difficult to increase a ratio of a surface area of a light-receiving section (photodiode) to an overall surface area of one pixel.
It is reported that, in a conventional CMOS image sensor using a 0.35 μm, single polysilicon layer/double metal layer CMOS process, a ratio of a surface area of a light-receiving section (photodiode) to an overall surface area of one pixel is 17% (see the following Non-Patent Document 1). It is also reported that, in a conventional CMOS image sensor using a 0.15 μm wiring-rule process, a ratio of a surface area of a light-receiving section (photodiode) to an overall surface area of one pixel is 30% (see the following Non-Patent Document 2). In the CMOS image sensor where the ratio of the surface area of the light-receiving section (photodiode) to the overall surface area of one pixel is 30%, a microlens is formed to condense light. This means that a CMOS image sensor having a small ratio of a surface area of a light-receiving section (photodiode) to an overall surface area of one pixel requires a light-condensing microlens.    Patent Document 1: JP 2000-244818A    Non-Patent Document 1: H. Takahashi, M. Kinoshita, K. Morita, T. Shirai, T. Sato, T. Kimura, H. Yuzurihara, S. Inoue, “A 3.9 μm Pixel Pitch VGA Format 10b Digital Image Sensor with 1.5-Transistor/Pixel”, ISSCC Dig. Tech. Papers, pp. 108-109, 2004    Non-Patent Document 2: M. Kasano, Y. Inaba, M. Mori, S. Kasuga, T. Murata, T. Yamaguchi, “A 2.0 μm Pixel Pitch MOS Image Sensor with an Amorphous Si Film Color Filter”, ISSCC Dig. Tech. Papers, pp. 348-349, 2005
It is therefore an object of the present invention to provide an image sensor having a sufficiently-large ratio of a surface area of a light-receiving section (photodiode) to an overall surface area of one pixel.